Objective: To study causes and implications of intraoperative conversion to thoracotomy during video-assisted thoracoscopic surgery (VATS) lobectomy. Methods:We performed an institutional review of patients undergoing lobectomy for known or suspected lung cancer with root cause analysis of every conversion from VATS to open thoracotomy. Results: Between 2004 and 2012, 1227 patients underwent lobectomy. Of these, 517 procedures (42%) were completed via VATS, 87 procedures (7%) were converted to open procedures, and 623 procedures (51%) were performed via planned thoracotomy. Patients undergoing thoracotomy were younger and had a higher incidence of prior lung cancers. Planned thoracotomy and conversion group patients had higher clinical T stage than patients in the VATS group, whereas the planned thoracotomy group had higher pathologic stage than patients in the other groups. Postoperative complications were more frequent in patients in the conversion group (46%) than in the VATS group (23%; P<.001), but similar to the open group (42%; P = .56). Validating a previous classification of causes for conversion, 22 out of 87 conversions (25%) were due to vascular causes, 56 conversions (64%) were for anatomy (eg, adhesions or tumor size), and 8 conversions (9%) were the result of lymph nodes. No specific imaging variables predicted conversion. Within the conversion groups, emergent (20 out of 87; 23%) and planned (67 out of 87; 77%) conversion groups were similar in patient and tumor characteristics and incidence of perioperative morbidity. The conversion rate for VATS lobectomy dropped from 21 out of 74 (28%), to 29 out of 194 (15%), to 37 out of 336 (11%) (P<.001) over 3-year intervals. Over the same periods, the proportion of operations started via VATS increased significantly. Conclusions: With increasing experience, a higher proportion of lobectomy operations can be completed thoracoscopically. VATS should be strongly considered as the initial approach for the majority of patients undergoing lobectomy.